Amplifier



w. 1.. cARLsoN AMPLIFIER Jan. 11,1927. 1,613,741

Filed August 17, 1922 RELATIVE VOLTAGE OUTPUT wAvs LENGTH METERS Inven'tor Wendell L. dam'z on i 7:01-11 ey Patented Jan. 11, 192 7.

/ UNITED STATES WENDELL L. CABLSON, OF WASHINGTON, DISTRICT OF COLUMBIA.

Application filed August 17, 1922. Serial No. 582,450.

My invention relates generally to radio frequenc amplifiers and more particularly to stabilizing the action and effectiveness of such amplifiers.

An object of the invention is to provide an amplifier in which the tendency to oscil late is reduced to a minimum.

Another. object is to increase the amplifying range over a wide band of frequencies. Heretofore it has been found necessary to take special precautions to prevent the amplifier from oscillating locally, or howlin by such means as connectingadjustab e bias potentials to the grids of certain of 1B the amplifying tubes, or other such similar arrangements, but with the'present invention the transformer characteristics are made quite uniform throughout their frequency range. I 80- y A further object of the invention is to provide a transformer construction, which will not amplify impulse energy such as static to a greater degree than the less damped signal ener A still further ob ect is to embody certain new combinations in the construction of inter-valve radio frequency transformers so as to allow the amplifier to operate with the utmost reliability without impairing its amplification characteristic.

With the above and other objects in view.

the invention consists in the combination, construction and arrangement of parts as will be hereinafter fully described. Reference is to be had to the accompanying drawings forming a part of this specification in which like reference characters indicate corresponding parts throughout the several views, and 1n which- I Figural is a schematic dia ram of a circuit employing three stages of amplification and a detectorc with resistance wound intervalve transformers, Fig. 2 is a modification of the radio transformer construction employing a short circuit winding,

Fig. 3 shows certain transformer characteristics plotted graphically, and

Figure- 4 is a modification of the radio transformer construction employing separate resistances in series with the win ings. l

Heretofore it has been customary to obtain .intervalve coupling by two general methods. First the resistance coupler and the impedance coupler both of which simply impress the alternating plate potential of one tube directly on the grid of an associated tube. Second the transformer coupler which transfers potential in the grid circuit of a second tube by magnetic coupling. This invention relates only to inter-valve transformers.

Referringmore particularly to Figure 1 of the drawings, the antenna primary circuit 5 is tuned to the incoming radio frequency signal. 6 is a secondary tuned circuit coupled to the primary circuit and connected to the grid 9 and filament f of the vacuum tube 7 The plate 9 of the tube is connected through the transformer primary winding 8 of the transformer T to a plate battery b. The winding 8 is magnetically coupled by the laminated iron core 9 to the secondary winding 10. The primary and secondary windings 8 and 10 are constructed of high resistance wire in order of- 10 to 20 times the resistance of an equivalent copper wire construction. For the purpose of illustrating schematically the transformer windings the inductance and resistance of the windings are shown separately and are'represented respectively by 8 and 8 for the pri mary and 10 and 10 for the secondary wind ing. The secondary winding 10 is connected to the input of tube 11. In like mannerv the tubes 11 and 12 are coupled to the inter-valve transformer 13. Tube 12 is also connected through transformer 14 with the The coil 19 represents a-short-cireuited turn of copper wire in the magnetic field of the transformer.

In the modification shown in Fig. 4, the

- windings 8 and 10 on core 9 are of ordinary copper or highly conductive wire but have resistances 21 and 20 inserted in series with these windings.

in Fig. 3 the characteristic curves of two radio frequency transformers are plotted with relative voltage output as ordinate against wave length in meters as abscissa. Uurve A is of a-transformer employing copper wire windings of low resistance and an iron core and curve'B is an identical transformer construction with the exception that resistance wire windings are employed. The double hump or two resonant peaks shown are due to employing iron cores.

The operation of the amplifier circuit of Figure l is as follows: A modulated radio signal impressed on the tuned antenna circuit 5 is transferred to the tuned coupled circuit 6 and thence to the first amplifying tube 7. Tubes 7, 11 and 12 are actuated successively by the signal energy, The amplification is governed by the tube constants, and the inter-valve transformer characteristics at the frequency of the signal being received. The signal is rectified in tube 15 and the modulated component actuates the telephones 16. z i i In radio frequency amplifiers there is a decided tendency for the individual stages to oscillate locally and for the several stages to oscillate collectively These oscillations are caused by capacity coupling between the high potential terminals of the circuit; resistance coupling in the common battery circuit; and irregular tranformer output voltage characteristics. Shielding the amplifier circuit is effective in reducing theca acity couplings of the circuits materially an radio frequency choke coils in the low potential side of the common battery circuit, when a common plate battery is used, are sometimes employed to eliminate the resistance cou ling effect.

ven with these precautions an amplifier employing inteiavalve transformers with characteristics as shown in curve A, Fig. 3, is likely to oscillate locally at say 210 meters which corresponds to the frequency at which the transformer responds most efficiently.

- 'In such cases it is necessaryito decrease the tube amplification to the point where oscillations cease. Usually this is-accomplished by varying the bias potential from a potentiometer, which is generally called a stabilizer. It is notdesirable from an operating standpoint to employ more than one vari-. able stabilizer adjustment and j if several stages are connected to the same stabilizer, the adjustment can be made correct for only one stage of amplification due to tlie variable characteristics of the tubes and slight irregularities in each transformer. In some cases, it is found necessary to provide fixed grid potential of some such valve as to decrease tubeamplification. Such a procedure will greatly impair the amplification obtained on wave lengths oif resonance such as 300 meters when employing transformers mercy/er istic, as illustrated by curves A and B, Fig, My invention makes it possible.

3, is used. to use iron cores, thus obtaining the wide range amplification due to the doublehumped characteristic obtainable in such a transformer. The series resistance or apparent resistance cuts down'the resonance voltage peaks and, in some cases, increases the valley voltages, thu's giving a more uniform output voltage throughout the transformer frequency range with a constant in put, and at the" same time the tendency to oscillate or howl is offset.

It is found in amplifiers employing transformers with characteristics as gra hically illustrated by curve B in Fig. 3 t at the correct grid bias voltage can be employed on all tubes for all frequencies throughout the amplifiers wave length range thus obtaining the maximum voltage-amplification from each tube. It should be understood that curves A and B are of individual transformers and that if the amplifier of Figure 1 were to employ transformers with curve A characteristics, it would be necessary to provide adjustable biases on the grids of successive tubes so as to reduce the amplification below the threshold oscillating value. The amplification would thus be about the same as obtained with an amplifier employing transformers with curve B charactenstics.

It has been found that with resistance winding transformers, or equivalents, that the highly damped impulses; that is, static interferences,' are not amplified to any greater degree than the signal ener while with a transformer whose characteristics are similar to curve A and when operating on a signal say at 300 meters, the impulse energy will be amplified to a greater degree than the signal.

It has been found practical to employ low arrangement of parts may be resorted to within the sec of the appendedclaims and series with at least one of the windings for without sacri ing any of the advantsges of increasing its resistance.

my invention, 2. In an amplifying transformer for radio Heying fully described my invention, what frequency currents, an iron core, primary 5 I desire to secure by Letters'Pu'tent is and secondary windings upon the iron core, 15

, 1. In an amplifying transformer for radio the primary and secondary windings confrequency currents, an iron core, primary sisting of low resistance wire and means in and secondary windings upon the iron core, series with each of the windings for increasthe primary and secondary windings coning the etfective resistance of each winding.

10 sisting of low resistance wire and means in i WENDELL L. CARLSON. 

